Subject-matter of the present invention are multilayer coating systems, their production, and the use.
In past years, great progress has been made in the development of acid- and etch-resistant clearcoats for automotive production-line (OEM) finishing. More recently there is now an increasing desire in the automotive industry for scratch-resistant clearcoats which at the same time retain the level achieved to date in their other properties too.
For instance, EP-A-540 884 discloses a process for producing multilayer coating systems, especially in the motor vehicle sector using free-radically and/or cationically polymerizable, silicone-containing clearcoats, the clearcoat being applied under illumination with light having a wavelength of more than 500 nm or in the absence of light, and the clearcoat layer subsequently being cured by means of high-energy rays. The surfaces obtained in this way are said to have good optical properties and a good scratch resistance.
Similarly, EP-A-568 967 discloses a process for producing multilayer coating systems, especially in the motor vehicle sector, using radiation-curable clearcoats.
DE 44 21 558, moreover, discloses a coating process in which a primer is partly crosslinked by means of radiation and then a radiation-curable coating material is applied. Subsequently, full crosslinking of the topcoat layer is carried out by means of electron beams. For additional and final curing of the coating material, furthermore, a concluding operation of heating the coating layer is recommended.
U.S. Pat. No. 4,675,234, finally, discloses a further multilayer coating system. In this case a basecoat is first of all applied to a substrate, and partly crosslinked by means of radiation, and a topcoat is applied to this partly crosslinked layer. This topcoat is subjected to radiation curing along with the basecoat layer.
The clearcoats employed to date do not adhere satisfactorily to the basecoats. In addition, the resistance of these multilayer systems to condensation is inadequate. Moreover, any reactive diluents present penetrate into the basecoat layer and so partly dissolve it. The consequence is an inadequately cured layer system.
The present invention is therefore based on the object of providing a multilayer coating comprising at least one basecoat layer and one topcoat layer and being free from the abovementioned disadvantages. The intention is, in particular, to achieve better adhesion of the base layer to the substrate and to the topcoat layer.
This object is achieved in that said coating can be produced by
A. applying a radiation-curable component (I) comprising binders based on polyurethane resins, polyacrylate resins, polyester resins and/or amino resins and also mixtures thereof to a substrate and subjecting it to thermal partial crosslinking or drying at temperatures of less than 100xc2x0 C., preferably 60-80xc2x0 C., and
B. applying a topcoat to this coating layer, and subsequently carrying out radiation curing.
Basecoat
Preferably, aqueous basecoats are employed. In so far as solvents are present, their content is preferably below 15%. Further essential features are that the basecoat is both radiation-curable and at least partially thermally crosslinkable, or drying, at temperatures below 100xc2x0 C. without irradiation.
Component I comprises binders which are at least partially crosslinking or drying at temperatures of below 100xc2x0 C., preferably at temperatures of 60-80xc2x0 C. In addition, the binders may be radiation-curable. The binders employed in accordance with the invention preferably have a double-bond density of 0.05-10, more preferably 0.1-4 and, with particular preference, 0.5-2 mol/kg.
The aromatics content lies preferably below 5%. The amine nitrogen content is preferably less than 2%, with particular preference below 1%.
It is likewise possible to employ non-radiation-curable binders comprising radiation-curable components having co-reactive groups, e.g. isocyanates, epoxides, melamines, malonic esters or anhydrides.
Radiation-curable oligomers may also be present in component I. In so far as the binders are not radiation-curable, such oligomers must be added.
In the basecoats of the invention the binders are employed preferably in an amount of from 5 to 90% by weight, with particular preference from 20 to 70% by weight, based in each case on the overall weight of the topcoat in the case of clearcoats or on the weight of the coating composition minus pigments and extenders in the case of pigmented systems.
As binders, use may be made of water-dilutable or water-dispersible polyurethane, polyacrylate, polyester and amino resins, and/or such resins which can be prepared in organic solution, and also mixtures thereof.